Signal irregularities during the operation of a video tape recorder/playback system of the type disclosed in pending U.S. Pat. application Ser. No. 388,929, filed Aug. 16, 1973, allowed July 7, 1975, may result in visually detectable disturbances as a result of the low tape speed and small recording headgap used in these systems. Similar problems occur in the use of video disc recorder/playback systems of the type disclosed in German Published Patent Specification DAS No. 2,216,100. The disturbances may result from defects in the recording medium or from loss or reduction of contact between the playback head and the tape or disc. Another possibility is a loss of a zero crossing when a video transition from sync black to white is recorded. The result is demodulation of the signal as if a very low frequency had been recorded instead of the high frequency of peak white as intended. Disturbances of short duration may be rendered virtually imperceptable, however, by the insertion of a compensating video signal to span the time period during which the signal is lost or an erroneous interpretation would otherwise result. One example of such a compensating signal is a grey-level signal. Another example would be a repetition of the preceding line through the use of delay lines as discussed in pending U.S. Pat. application Ser. No. 507,433, filed Sept. 19, 1974.
In order to insert the compensating signal at the appropriate times it is known to use a detector to sense signal defects and to trigger a compensating signal generator. Some known detectors, however, are of the envelope type which only sense the dropping of the signal amplitude below a specified level. An example of such a detector is disclosed in U.S. Pat. No. 3,328,521. This type detector performs satisfactorily on the first class of problems mentioned above when the signal is lost due to a tape defect or reduction of contact pressure. The envelope type detector, however, is not capable of detecting the second class of problems, for monitoring the amplitude of the signal does not detect the loss of a zero crossing.
In the above referenced German Published Patent Specification DAS No. 2,216,100, a circuit for suppression of distortions during recording or playback of FM-modulated video signals in a video recording/playback system is disclosed. Such distortions consist of a loss of at least one portion of the FM-modulated video signal as occurs for instance, in a loss of zero crossings. To accomplish the suppression, this known circuit uses a special high pass filter inserted before the demodulator in the playback channel. The filter has a frequency limit lying lower than but close to the lowest frequency of the side bands of the modulated carrier frequency to be transmitted by the recording/playback system. This known circuit is only useful in suppressing such distortion and cannot be used to detect and compensate and cannot be used for dropouts and zero crossing defects.
A drop out detector is required, therefore, to detect all classes of signal irregularities. The present invention is such a drop out detector.